The electrolysis industry represented by alkali chloride electrolysis has an important role in the material industries. Alkali chloride electrolysis, while significant, consumes much energy, and energy savings therefor has become a world-wide concern. Taking alkali chloride electrolysis as an example, for the purpose of achieving energy savings as well as solving environmental problems, transitions have been made from a mercury process to a diaphragm process and then to an ion exchange membrane process, thus affording an energy savings of about 40% over a period of about 25 years. Notwithstanding the foregoing, the power cost is 50% of the total manufacturing cost such that there is a great need for further energy savings. However, as far as present processes are concerned, manipulations for achieving additional energy savings have almost reached the highest possible level. A fundamental change, such as a change in the electrode reaction, must be made before further energy savings can be realized. Under these circumstances, application of a gas diffusion electrode used in fuel cells is the most promising of conceivable means for producing power savings.
The cathodic reaction using a conventional metal electrode, represented by reaction formula (1), is changed to reaction formula (2) when a gas diffusion electrode is used as a cathode. EQU 2NaCl+2H.sub.2 O.fwdarw.Cl.sub.2 +2NaOH+H.sub.2 ;E.sub.O =2.21 V(1) EQU 2NaCl+1/2O.sub.2 +H.sub.2 O .fwdarw.Cl.sub.2 +2NaOH;E.sub.0 =0.96 V(2)
That is, replacement of a metal electrode with a gas diffusion electrode reduces the potential from 2.21 V to 0.96 V, theoretically affording energy savings of about 65%. Hence, various studies have been directed toward practical implementation of alkali chloride electrolysis using a gas diffusion electrode.
A gas diffusion electrode generally has a semi-hydrophobic or water-repellent structure, which is composed of a hydrophilic reactive layer having supported thereon a catalyst (e.g., a platinum catalyst) and a water-repellent gas diffusion layer joined together. Both the reactive layer and the gas diffusion layer are formed using polytetrafluoro-ethylene (hereinafter abbreviated as PTFE) as a binder resin. The properties of PTFE are advantageously used by incorporating the PTFE in a high proportion in the gas diffusion layer and in a low proportion in the reactive layer.
Application of the gas diffusion electrode to alkali chloride electrolysis gives rise to several problems. In order for a gas diffusion electrode to work properly as a cathode in alkali chloride electrolysis, ample oxygen-containing gas should be supplied to its surface. If the gas supply is insufficient, the reaction at the cathode is accompanied by the evolution of hydrogen. It follows that a large quantity of energy is consumed and, in some cases, there is a danger of explosion with the oxygen-containing gas thus supplied. If an adequate amount of an oxygen-containing gas is supplied to the cathode, a water producing reaction between hydrogen ion and oxygen takes place on the cathode to achieve satisfactory energy savings. Thus, the high performance of a gas diffusion electrode is only revealed under limited conditions. However, few investigations have been carried out to identify the electrolysis conditions that promote the high performance of a gas diffusion electrode, especially the manner of supplying gas to the cathode surface. For example, an oxygen-containing gas must be supplied to the cathode surface not only in sufficient amount but also uniformly. If not, the gas diffusion electrode would have a portion that functions properly and a portion that does not, thus failing to achieve its full function as a whole. Non-uniformity in the gas supply tends to occur when the temperature of a member of the electrolytic cell varies or when there is a difference in temperature between a humidifier and the electrolytic cell.
In an electrolytic cell using a gas diffusion electrode for producing an alkali hydroxide, moisture is typically mixed with an oxygen-containing gas to thereby supply a moistened oxygen-containing gas for the purpose of adjusting the concentration of the alkali hydroxide thus produced.
While the optimum concentration of catholyte in the general electrolysis of sodium chloride (brine) by an ion-exchange membrane process varies depending on the kind of ion-exchange membrane, it is considered to be 30 to 35% for, e.g., sodium hydroxide production. The sodium ion migrating through an ion-exchange membrane from an anode chamber to a cathode chamber is accompanied by about 3.5 to 4.0 molecules of water. With the accompanying water, the sodium hydroxide produced in the cathode chamber will have a concentration of about 42%. If the sodium hydroxide concentration exceeds 35%, the ion-exchange membrane will have an increased electric resistance, resulting in an increase in voltage and a decrease in the life of the ion-exchange membrane. The water deficiency is 1 to 1.5 molecules, in terms of water accompanying sodium, per sodium molecule. The number of deficient molecules is 2 to 3 times that of the oxygen gas that is supplied.
The concentration of the sodium hydroxide produced in the cathode chamber can be optimized by mixing the oxygen-containing gas supplied to the cathode chamber with high-temperature saturated steam, and supplying the thus moistened gas to the cathode chamber while maintaining that temperature. This method tends to cause condensation with a slight change in temperature. Water may be supplemented in the form of mist. In this case, too, a water supply gradient tends to be generated, and it is difficult to uniformly supply water over the whole surface of the gas diffusion cathode.
These disadvantages are liable to occur particularly in a large-sized electrolytic cell, causing a bottleneck in applying an electrolytic cell using a gas diffusion electrode to the industrial production of an alkali hydroxide.